Antenna



May 16, 1950 Fig.!.

A. ALFORD ANTENNA Filed Ma 15, 1946 A A L A A ANDREW ALFORD I N V EN TOR.

ATTORNEY Patented May 16, 1950 UNITED STATES PATENT OFFICE ANTENNA Andrew Alford, Cambridge, Mass. Application May 15, 1946, Serial No 669,758

25 Claims. (Cl. 250-33) This invention'relates to new and useful improvements in radiators for vertically polarized waves. v

Among the objects of this invention are: radiators and supporting structures, e. g. masts, of simple, inexpensive and yet rugged construction; small structures which are easy to install and maintain in service; and others whose nature will more clearly appear from a consideration of the problems that confronted the workers in this art, followed by a description of the way in which I have solved them.

Radio telephone, telegraph, facsimile, etc., communication, e. g. between fixed stations and mobile stations installed in automobiles, buses, etc., is usually effected on vertically polarized waves which have somewhat greater field intensity near ground and can be received on and transmitted by so-called whip antennas customarily employed on automobiles.

The fixed stations are usually provided with guyed steel tubular masts about 200 or 250 feet high and having a diameter of two to three and one-half inches within six to ten feet from the top and somewhat greater below, but usually not great enough to make the mast self-supporting. Therefore, sets of wire rope guys or the like are employed for holding the mast upright, selfsupporting structures being too expensive.

A well known type of transmitting antenna which is mounted above the top of the mast consists of a verticalquarter-wave stub-connected to the inner conductor of a coaxial transmission line, in combination with a quarter-wave tube. The upper end of the tube is connected directly. to the outer conductor of the coaxial line. The lower end of the tube is insulated from the outer conductor so that high frequency currents flowing on the outer surface of the tube cannot continue directly down the mast but must pass through the space between the tube and the outer conductor. This arrangement is calculated to provide choking action to reduce the currents in the mast.

- Among the shortcomings of such antenna are the following:

The choking action provided by the quarterwave tube arrangement is insufficient to produce,

the required reduction in the mast currents which, therefore, will radiatea substantial portion of the total power. the mast may or may not be in proper phase with the antenna radiation in the horizontal direction, so that there may be a minimum of the total radiation in the horizontal directions and most of the power may be scattered in useless directions.

The power radiated by.

To rectify this undesirable condition, one or.

two additional quarter-wave chokes are frequently provided at suitable distances below the antenna. Since these chokes must be sharply tuned and are themselves radiating, the radiating system, consisting of the antenna and chokes, is multiply tuned and must be adjusted for proper relative phases and for proper choking action.

The antenna cannot be mounted below the top of the mast. It is, therefore, impossible to increase the gain by stacking a number of such antennas.

Attempts have been made to design antennas consisting of two quarter-wave sleeves or of two half-wave sleeves that could be installed concentrically around the mast. However, two quarter-wave sleeves are too sharply tuned and have a very low input impedance. Antennas con sisting of two half-wave sleeves require a balanced high impedance feed of the order of 350,

ard coaxial transmission line.

I. cross section taken in a plane through the axis In accordance with the present invention, the radiator or radiators are attached to a hollow body, in whose wall a transmission line is formed,

the body itself constituting a radiator and an impedance-matching means between the feeder and the radiator or radiators.

One feature of the invention is that the trans mission line is formed by a narrow gapof the order of a small fraction of a, wave length-in the wall of the body. The edges of this gap are preferably sharp and they form the transmission line to which the feeder is connected.

In different embodiments of this invention the.

bar bridges the mid-points of the edges of the gap.

In accordance with the present invention, the

hollow body may be fastened to a metal mast;

centrally with respect to the transmission line, without suffering the disadvantages to which I have referred.

In accordance with the present invention, the dimensions of the body are such that transverse In such embodiments it would not of the'mast of the air space within said body on one side of the mast will have an area of about one-hundredth of a wave length squared, .0105A It is noted in this connection that the antenna is particularly suited for operating on a band of frequencies and that whenever, in the following description and claims, reference is made to wave length or frequency, the center, frequency of said band is meant, unless, of course, a different meaning is indicated.

In the preferred embodiment here disclosed, the body is a cylinder in which the transmission line is formed, in accordance with aieatureof this invention, in the form of a gap in the outside wall in a plane transverse to the axis of the cylinder and is short-circuited opposite its feed point. The cylinder has preferably a circular horizontal cross section, but this is not essential for all applications ,or the invention to good advantage. It should be understood, therefore, that the Word cylinder as used in the description and claims, without further qualification, is intended to cover all hollow bodies suitable for the purposes of the present invention. The cylinder serves as a radiator, a mechanical supporting means for attachment to a mast or the like, and an impedance-matching means. For a frequency band whose center is 378 mc., a cylinder 2%" high and 10" in diameter which has a 1% wide gap in a plane transverse to its axis, and which is axially traversed by a 1 Steel tube through its center produces the desired results when equipped with four 7.5" radiators at tached to each cylinder half and parallel with the axis of the tube, pairs of which project from pposite edges of said gap away from each other.

The nature of the invention willbest be understood from the following description of anembodiment thereof as applied to the drawings in which Fig. 1 is a perspective of the antenna structure, and Fig. ,2 a curve of the antenna on a 52- ohm line. I n V k As shown in Fig. l, the antenna c'ons ists of two circular body elements I, 2 Which may be castings or spinnings shaped like pie plates. The body elements consist of bottom discs 3', l having upstanding rims (bearing theabove-mentioned numerals I, 2) facing one another. These body elements I, 2 are joined by means of a metal tube which is mechanically attached to inside surfaces of bottoms 3 and 4 of elements I and 2 at substantially their centers. In practice, tube 5 may form part of either element I or 2, so that the assembly consists of only two pieces. Accordingly tube 5 may be made of a metallic and, as a result, of a conductive material, However, as will be obvious after the complete description of this device,the inside surface of tube 5connects areas of Zero (or extremely low) RF voltage of elements I audit, and may, therefore, be non-conductive. While the inside of tube 5 need not beconductive for this device to operate, on the other hand it need not be non-conductive to keep any supporting structure, such as a mast through tube 5, from becoming energized with,

RF voltages. However, the outer surface of tube 5 should be conductive as there must be a low impedance axial connectionbetween the inner surfaces of elements I and 2 and, if desired, a metallic mast may be used directly to perform this function. Tube 5 is of such length that body elements I, 2 do not touch each other but are held. so that their bottom discs 3 and] are parallel to one another and their rims areseparated by gap 6. This gap is short-circuited by metal band 7. Bottoms 3 and 4 have centrally located holes which register with the ends of the hole through 5. The structure I-I may be described as a hollow body or cylinder whose ends are closed by discs 3 and 4, and whose side wall has a gap 6 which runs around the cylinder in a plane parallel to and midway between the discs 3' and 4, and returns on itself. The cylinder halves are held together by coaxial tube 5 fastened to the inside of discs 3 and 4. The structure of the hollow body or cylinder may be of material such as solid sheet metal or solid sheet plastic (with a conductive surface adhering thereto to achieve obvious electrical properties) and accordingly the interior space will be bounded by wall surfaces unbroken except for gap 6. However, this is not necessary. This device may alterna ively be made ofscreen wire, of perforated sheet metal, or of perfdrate'd sheet plastic with a conductivesurface: If theopenings are not too large with respect to cm; wave length corresponding to the highest dperating frequency, e. g. less than six one-hundredtlis of a wave lengththe inner wallswill be eliectively unbroken electrically. Formulae relating to the limiting dimensions of such permissible openings are available in the literature; e

The antenna is energized over'a coairial trans mission line having an outer conductor 8 aridinner conductor 9. The inner conductor 9 is coiinected to the rim of one of the two body elements, for example plate I, and the outer use: ductor 8 is connected to the rim of the other. These connections are made at points diametr'i' cally opposite each other onga'p 6 on the s'de' of the hollowcylinderopposite to the short-err cuiting band 1. H v

Four metal radiators formed as rods I0, I I; I2, I3 are conductively connected to the rim of element I and four similar rods I4, I 5, I6, I? are connected to the rim of element 2". These two sets of rods project away -ffrom bottoms 3 and 5 respectively at right angles to them and, therefore, parallel to the axis of tube 5. 7

When the ante nna ismounted on a steel mast I8, the latterpass'es through the tube 5 and the holes in bottoms 3 and}; The cylinder -l'-f- 1 is prevented from sliding user down mast I8 by suitable collars or clamps such as Ht-28. The coaxial transmission line 8, "9 may be suitably clamped to mast I 8 and to element 2.

Whe n the antenna energized, the adjacent edges of the cylinder halves, i. e. the edges of I', 2 forming gap 6;, constitute a balancedtransmission line. This line isloaded a distributed reacta'ric which depends on the area of a transverse cross section, taken in a plane through the axis of mast- I8, of the air spa'ceenclosed on one side of tube 5 between the inner surfaces of elei nehts I, 2 and the outer surfac of tube "5'. If the inside radius of the hollow cylinder'is designated-ash radius of tube e as r, and the distance b ett es the inner surfaces of bottoms 3' and 'as this cross-sectional area may becalcul the formula A?(R VT)'H. grmese l cn'sio'ns are convertedinto wave lengths and the farms solved, A=.0l05}\ I,

Short-circuiting band I produiie's' mammaplete reflection of waves traveling along ga s, and the interference between direct traveling waves; and reflected traveling' waves resultsin ;a standing wave distribution of voltage across gapfi. One minimum of voltage across the gap occurs at short-circuiting band 7; two 'r'iiaxi ma of voltage occur at points on gap 6 on opposite sids of'the hollow cylinder each of which is about between I and the diametrically opposite point where the coaxial cable connection is made. Two additional voltage minima under certain conditions may occur a short distance from and on each side of the point of connection of the concentric line.

Substantially one-half of the respective voltage differences between the edges of gap 6 at the points where radiators III, II, l2, l3 I! are connected are applied respectively to one end of each radiator, causing them to operate as vertical radiators. Since the voltages existing across the gap between minima are substantially in the same phase for different points along that portion of the gap, and since, when this device is'properly proportioned and adjusted, the only points of minima are in the region of the feed point and at the shorting band, all the upper rods Ill-l3 are energized in the same phase and all lower rods l4-l I are energized in the opposite phase.

It is noted that the second minima need not coincide with the feed point and that preferably they should be at imaginary points beyond the feed point rather than between it and the shorting band.

Thus, hollow cylinder l'l serves as a means for obtaining balanced, equal and opposite voltages and, at the same time, it operates as a transformer which matches the combined impedances of radiators l-l'i to the impedance of the coaxial transmission line feeder 3, 9. In addition, the cylinder serves as a supporting means for radiators Ill-11.

Typical proportioning of the several parts of the antenna may be as follows: D=.32/\, d=.0327\, H=.072)\, g=.0048)\, L=.236 t. The dimensions are here as well as in the drawings given in terms of the operating wave length at the frequency which is at the center of the band of frequencies. The antenna so proportioned is substantially matched to a 50 ohm coaxial transmission line within said frequencies and the second voltage minima are properly located. The dimensions in inches would be as follows at the geometric means frequency of 3'78 rnc.: D=", d=l, H 2T36 !I g i:; s!r L 7-5II- Fig. 2 shows a measured standing wave ratio as a function of frequency for the antenna having the above proportions.

The following considerations control the proportioning of the antenna. The voltage distribution along the gap 6 should be such that the radiators l0-l1 are energized in the same phase. Since the phase of the voltage varies rapidly in the region of a voltage minimum except at the short-circuiting band 1 and the feed point, it is desirable to have the proportions and electrical behavior of the cylinder such that there are no other minima or that any such other minima of voltage occur between any radiators near the feed point and the feed point, or even at stillgreater distances from the short-circuiting band I even to imaginary points beyond the feed point. When voltage minima occur in the neighborhoods of two pairs of radiators, then the voltages applied to the other pairs of radiators are higher than the voltages applied to the first pairs. This condition is ordinarily not desirable because it causes some asymmetry in the horizontal radiation pattern, distorting it from a circle. More desirable proportioning is obtained when the distance from band I to the first voltage minimum or to the first imaginary minimum along the gap in each direction around the hollow cylinder is equal to (or is the cylinder because, then the voltages applied to each pair of radiators located between the shorting band andsuch a minimum are nearly equal.

If, moreover, the two sides of each cylinder half havestanding waves of equal magnitude, this will result in a substantially circular radiation pattern.

The voltage distribution along the gap 6 depends on two factors: The circumference of the hollow cylinder, i. e. the gap length, and the velocity of propagation along the edges of the gap. The gap length can be increased by increasingthe propagation velocity. The propagation velocity is controlled by the cross-sectional area'A and the capacity per unit length between the edges of the gap. A cross-sectional area, A=.0105 to-" gether with a gap which is a small fraction of a wave length wide (9048A), and is formed by thin metal edges, results in a velocity of propagation along the gap about 20 greater than the velocity in free space. This permits the use of a hollow cylinder of large circumference and results in a.

good impedance match.

The effect of the addition of radiators on the voltage distribution along the gap depends in part on their length in comparison with the operating Wave length. When each of the individual radiators is approximately one-quarter wave length long and the input impedance of each radiator is resistive, there is little effect on the distances between the first voltage minima and short-cir-c cuiting band 1.

Below the center of the band of operating fre quencies, the radiators are less than one-quarter wave length long and the impedance of the radiators is capacitative. The effect of shunting a capacitative impedance across the gap is to decrease the velocity of propagation and, therefore, to

shorten the distance between short-circuiting band 1 and the first voltage minima.

Above the center frequency, the radiators are therefore, to increase the distance from band I to the first voltage minima.

I The over-all effect of the radiators is thus beneficial in that at frequencies below the center frequency of the antenna the capacitative impedance of the radiators lowers the effective propagation velocity which, without radiators, due to changes in the behavior of the hollow cylinder when frequency changes, would be too high. At frequencies above the center frequency the induc tive impedance of the radiators accelerates the velocity of propagation which, without radiators, would become too low. This compensating eflect ofthe radiators is important in maintaining the desirable voltage distribution across the gap over a wide band of frequencies. The nearly constant voltage distribution assures the application of nearly equal relative voltages to the four pairs of radiators and results in a, substantially circular horizontal pattern which is maintained over a Wide band of frequencies. Furthermore, the input impedance presented to the coaxial feeder is maintained near the desired value of 50 ohms over a. substantial frequency band, because the positions or imaginary positions of the voltage minima remain nearly constant with respect to the feed point.

In the above discussion of the theory of operation of the antenna it has been indicated that the greater than) one-half of the circumference of cylinder'halves need not be of solid metal, but.

in a plane transverse to its longitudinal axis and interrupts its conductivity midway between its ends, a feeder for said line comprising a concentric cable connected with the edges of said gap, a short-circuiting means for the transmission line conductively connecting the edges of the gap at a point diametrically opposite the point of connection of the feeder and producing a standing wave distribution having one minimum at the short circuit, another minimum at the point of connection of the cable, and two maxima between said minima on opposite sides of the cylinder and an equal number of rods amounting to at least four attached to each edge of said gap projecting substantially a quarter wave length from each end of the cylinder parallel with its axis and so positioned along said gap that nearly equal relative voltages are applied thereto, and that substantially equal halves of the voltage differences existing across the transmission line at points where rods are attached are applied to ends of opposed pairs of rods attached to opposite edges of the gap whereb the rods operate as vertical radiators, the rods projecting from one end being energized in one phase and the other rods in the opposite phase.

9. In an antenna for vertically polarized waves, a conducting cylinder having a large diameter compared to its height, conducting discs closing the ends of the cylinder, said cylinder having a narrow gap in a plane parallel with and midway between the discs and extending around the side wall, a short-circuiting bar across the gap, a conducting connection vaxially located within the cylinder between the discs, a feeder connected with the edges of the gap at points diametrically opposite said short-circuiting bar, and metal rods fastened to the edges of the gap and to each cylinder half and projecting therefrom parallel with the axis of the connection.

10. In an antenna for vertically polarized waves, a cylinder having conducting inner surfaces, conducting discs closing the ends of the cylinder, a conducting tube axially located within the cylinder and conductively connecting the gap diametrically opposite said short-circuiting a volume of space .072A high and having a diameter of 32x, cross sections of said cylinder in planes transverse to its axis being circular, centrally perforated metal discs closing the ends of the cylinder, the side wall of the cylinder having a' gap 0048i wide extending around the cylinder in" a plane parallel with and midway between the discs, a short-circuiting bar across the gap, a metal tube having an outer diameter of .032A axially located within the cylinder and metallically connecting the discs, said tube having the ends of the annular channel through it in alignment with the perforations, a concentric cable having an outer conductor connected with one-half and an inner conductor connected with the other half of the cylinder at points on the edges of said gap which are diametrically opposite said short-circuiting bar, 'a hollow metal mast projecting through said tube and perforations supporting said cylinder, and four metal rods fastened to each edge of the gap at substantially equally spaced intervals at points where nearly equal relative voltages are applied thereto, said rods projecting away from the corresponding discs and parallel with the mast toa tially defining a toroidal shaped space, said hollow conducting body forming an air gap extending about the toroidal shaped space on its outer side forming with the conducting materialadjacent either side of the air gap a transmission line and a feeder having one line connected to'the conducting material adjacent the air gap on one side and another line connected to the conducting discs, the cross-sectional area of the toroidal space between the outside of the tubeand the inner surfaces of the cylinder being substantially .0105k said cylinder being separated into two parts to define a non-conducting gap substantially .0048x wide around the side wall of the cylin-' der in a plane parallel with and midway between the discs, a short-circuiting bar across the gap, a feeder electrically connected with the edges of the gap at points diametrically opposite said shortcircuiting bar, and metal rods electrically connected to each edge of the gap and projecting in space parallel with the axis of the tube to a distance of substantially one-quarter wave length.

11. In an antenna for vertically polarized waves, a metal cylinder having an internal diam-'- fraction of a wave length wide in a plane parallel with and midway between the discs extending around the side wall of the cylinder, a short-circuiting bar across the gap, a metal tube having substantially one-tenth the diameter of the cylinder axially located within the cylinder and metallically connecting the discs, a concentric cable having an outer conductor connected with one edge of the gap and an inner conductor connected with the other edge of the gap at a point on said on the other side of the air gap.

material opposite the connection of the first line 14. In an antenna, a hollow conducting boi y having its internal surface boundaries substantially defining a toroidal shaped space, said hollow conducting body forming an air gap extending about the toroidal shaped space on its outer side forming with the conducting material adjacent either side of the air gap a transmission line and a feeder having one line connected to the conducting material adjacent the air gap on one side and another line connected to the conducting material opposite the connection of the first line 1 on the other side of the air gap, and a short circuit member connected across the air gap spaced away from the position where the feeder is coning material opposite the connection of the first line on the other side of the air gap, and a short circuit member connected across the air gap diametrically opposite the position where the feeder all i is connected tothe conductive material at either side of the air gap.

16". In' an antenna, a hollow conducting body having. its internal surface'boundaries substantiallydefining a toroidal shaped space, said hollow conducting body forming an air gap extending about the toroidal shaped space on its outer side forming with the conducting material adjacen't'eitherside of the. air gap a transmission line 'anda feeder having one line connected to the conducting material adjacent the air gap on one side and another line connected to the conducting 'material opposite the connection of the first line "onithetother side of the air gap and a plurality of radiators projecting "from the conductive body at spaced intervals adjacent the air gap.

v1"I;jIn' an antenna, a hollow conducting body having its internal surface boundaries substandefining a toroidal shaped space, said hollow conducting body forming an air gap extending about the toroidal shaped space on its outer side formingtwith the conducting material adjacent either sideof the air gap a transmission line and a feeder. havingone line connected to the conducting material adjacent the air gap on one side and another line connected to the conducting material opposite the connection of the first line on the other side of the air gap, a short circuit member connected across the air gap spaced away from the position where the feeder is connected to'jthe conductor material at either side of the air gap;and aplurality of radiators projecting from V the conductive body at spaced intervals adjacent the air 'gap.

'18..In'an antennapa hollow conducting body having its internal surface boundaries substantially defining a toroidal shaped space, said holfirst line on the other side of the air gap, a short circuitmember connected across the air gap diametrically opposite the position where the feeder is connected to the conductive material at either side of the air gap and'a plurality of radiators P ojecting from the conductive body at spaced intervals adjacent and on opposite sides of the air gap.

'19. In an antenna, a hollow conducting body having its "internal surface boundaries substantially defining a toroidal shaped space, said holflow conducting body forming an air gap extending about the toroidal shaped space on its outer side forming with the conducting material ad- 'jacent "either side of the air gap a transmission line and a feeder having one line connected to the conducting material adjacent the air gap on; one side and another line connected to the conducting material opposite the connection of the'first line on the other side of the air gap and a plurality of radiators projecting from the conductive body at spaced intervals adjacent the air gap, said air gap lying substantially in a plane andthe radiators extending normally to said "plane.

"20. In an antenna, a hollow conducting body having its internal surface boundariessubstantially defining a toroidal shapedspacaxsaid hollbwcbnducting body forming an air gap extend- "ing about thetoroidal shaped space on its outer side forming with the conducting material ad- V 12 jacent either side of the air gap 2. transmission line and a feeder having one line connected to the conducting material adjacent the air gap on one side and another line connected to the conducting material opposite the connection of the first line on the other side of the air gap, and a support for said conducting body passing transversely through the same.

21. In an antenna, a hollow conducting body having its internal surface boundaries substantially defining a toroidal shaped space, said hollow-conducting body forming an air gap extending about the toroidal shaped space on its outer side forming with the conducting material ad- 'jacent either side of the air gap a transmission line and a feeder having one line connected to the conducting material adjacent the air gap on one side and another line connected to the conducting material opposite the connection of the first line on the other side of the air gap,-a short circuit member connected across the air gap diametrically opposite the position wherethe feeder is connected to the conductive material at either side of the air gap, a plurality of radiators projecting from the conductive body at spaced :intervals adjacent and on opposite sides of the air gap and a support for said conducting body passing transversely through the'same.

22. In an antenna, a hollow conducting body having its internal surface boundaries substantially defining a toroidal shaped space, said hollow conducting body forming an air gap extending about the toroidal shaped space on its outer side forming with the conducting material adjacent either side of the air gap a transmission line and a feeder having one line connected to the conducting material adjacent the air gap on one side and another line connected to the conducting material opposite the connection of the first line on the other side of the air gap, a short circuit member connected across the air gap diametrically opposite the position where the feeder is connected to the conductive material at either side of the air gap, said air gap lying substantially'in a plane, a plurality of radiators projecting from the conducting body at spaced intervals adjacent and on opposite sides of the airgap and perpendicular to said plane and a conductive mast passing through the conducting body and parallel to said radiators.

'23. In an antenna, a hollow conducting body having its internal surface boundaries substantially defining a toroidal shaped space, said hollow conducting body forming an air gap extending about the toroidal shaped space on its outer side forming with the conducting material adjacent either side of the air gap a transmission line and a feeder having one line connected to the. conducting material adjacent the air gap on one side and another line connected to the conducting material opposite the connection of the first line on the other side of the air gap, a short circuit member connected across the air gap spaced away from the position where the feeder is connected to the conductor material at either .side of the air gap and means including said conducting bodyfor electrically loading said line withdistributed reactance.

24. In an antenna, a hollow conducting body having its internal surface boundaries substantially defining a toroidalshaped space, saidhollow conducting body forming an air gap extending about the toroidal shaped space on its outer side forming with the conducting material adjacent either side of the air gap a transmission line and a feeder having one line connected to the conducting material adjacent the air gap on one side and another line connected to the conducting material opposite the connection of the first line on the other side of the air gap, a short circuit member connected across the air gap spaced away from the position where the feeder is connected to the conductor material at either side of the air gap, and a plurality of radiators projecting from the conductive body at spaced intervals adjacent the air gap, said antenna adapted to transmit with substantially uniform amplitude a short wave freqency band with a resistive input impedance at the center frequency, a 'capacitative input impedance below and an inductive input impedance above the center frequency.

25. In an antenna, a hollow conductin body having its internal surface boundaries substan tially defining a toroidal shaped space, said hollow conducting body forming an air gap extending about the toroidal shaped space on its outer side forming with the conducting material adjacent either side of the air gap a transmission line and a feeder having one line connected to the conducting material adjacent the air gap on one side and another line connected to the 14 conducting material opposite the connection of the first line on the other side of the air gap, a short circuit member connected across the air gap spaced away from the position where the feeder is connected to the conductive material at either side of the air gap, a plurality of radiators projecting from the conductive body at spaced intervals adjacent and on opposite sides of the air gap whereby substantially equal relative but oppositely phased voltages may be applied to the radiators projecting from the transmission line gap in substantially the same direction.

ANDREW ALFO-RD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,199,635 Koch May 7, 1940 2,234,293 Usselman Mar. 11, 1941 2,235,506 Schelkunoff Mar. 18, 1941 2,238,770 Blumlein Apr. 15, 1941 2,349,942 Dallenbach May 30, 1944 2,404,196 Seeley July 16, 1946 

